Three-dimensional turbulence modeling for free surface flows

Abstract

In engineering practice, some of the hydraulic structures encountered are used for distribution or collection of flows, measurement of flow rates or regulation of flows. It is very important to know the flow characteristics such as the mean flow pattern, the velocity distributions, and the pressure distribution in closed conduits or the water surface profiles in open channels. These flow characteristics can be obtained using experimental methods, exact theoretical methods or numerical methods. Existing commercial codes have some limitations. For instance, the memory requirements and execution time are often excessive for solving hydraulic engineering problems. The boundary conditions to be used are rigidly specified in the software. The CFD software is also very expensive. It is beneficial to develop and validate a new numerical model that can speedily solve the specific problems in hydraulic engineering as mentioned earlier. The present study develops and validates an efficient and accurate numerical turbulence model to simulate common hydraulic problems. The three-dimensional governing equations include the Reynolds-averaged Navier-Stokes (RANS) equations and the two-equation turbulence models such as the k-} model and the k-&epsiv; model. The Volume of fluid (VOF) scheme is also incorporated to capture the free surface profiles in open channel flows. For solving typical problems, the model developed presently needs much shorter execution time and requires less memory than the commercial code. For instance, to simulate open channel junction flows, the general-purpose commercial code (FLUENT) program required 30 days on the PC to reach the solution. The present model required about 4 days of execution time to solve the same problem. To begin with, dividing and combining flows in rectangular closed conduits are simulated, as these flows are simpler than their counterparts in open channel flows. These simulations are later extended to two and three-dimensional flows that have free surfaces. The simulation results provide pressure distributions in closed conduits and water surface profiles in open channels besides velocity distributions and flow patterns. The results also yield zones of flow separations in expanding flows and stagnation zones in dividing and combining flows. The predictions of the simulations are validated using existing test data. Especially, for three-dimensional dividing flows in open channels, new test data are also obtained for the present model validation. For the lateral weir flows, the nonlinear partial least square method is used to obtain the functional relationship among the principle weir flow parameters.